1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine and a printer, and more specifically, relates to an apparatus that has a mechanism to straighten a curled sheet.
2. Description of the Related Art
Some known image forming apparatuses, such as copying machines and printers, are capable of reversing and ejecting a sheet after forming an image on the sheet and/or are capable of forming images on both sides of the sheet. Such image forming apparatuses include a mechanism for reversing and ejecting the sheet and/or a delivering mechanism for reversing the sheet so that an image can be printed on one side of the sheet after an image has been printed on the other side.
Such a known image forming apparatus having such mechanisms comprises an image forming unit including a photoconductive drum, a charging unit disposed in the circumference of the photoreceptor drum, a developing unit, and a transferring unit. When forming an image on a sheet, the sheet is supplied from a paper-supplying unit to the image forming unit, where a toner image is transferred to the sheet. Then the sheet is sent to a fixing unit, where the unfixed toner on the sheet is fixed by heat and pressure. Finally the sheet is ejected from the image forming apparatus.
If an image is also to be formed on the other side of the sheet, the sheet having an image on one side is sent to a reversing path. Subsequently, the sheet is reversed and sent to a reversed-side path so as to deliver the sheet to the image forming unit again. The reversing path is curved and connected to the reversed-side path. The curvature of the reversing path is maximized so as to minimize the width of the image forming apparatus.
In the reversed-side path of such an image forming apparatus, a horizontal-registration detection unit for detecting the alignment of the sheet in the direction orthogonal to the delivery direction of the sheet (hereinafter this direction is referred to as the “width direction” of the sheet) and delivery rollers are provided. To form images on both sides of a sheet, after an image is formed on one side of the sheet, the front edge of the sheet is disposed against the delivery rollers to correct skew. Then the horizontal-registration detection unit detects the edge of the sheet in the width direction. According to the detected results, the position of the image to be formed on the second side of the sheet is adjusted.
An image-detection unit for detecting the color density and color of an image is disposed on an ejection unit of the image forming apparatus for ejecting the sheet on which an image has been formed by the image forming unit. The image-detection unit is provided so as to constantly maintain the color density of the images formed on sheets of paper at an appropriate value. After a reference image having a predetermined color density is formed on a sheet at the image forming unit, the color density of the reference image is detected by the image-detection unit. The measured color density is compared with the desired color density so as to prepare a conversion table. This conversion table is used to control the color density of the image by converting the color density characteristics of the image data in accordance with the conversion table.
However, in the above-described image forming apparatus, when a sheet is delivered through the fixing unit, the sheet curls due to the heat and pressured applied.
As disclosed in Japanese Patent Laid-Open No. 11-199142, to straighten the sheet curled at the fixing unit, a curl-corrector is disposed directly after the fixing unit. The curl-corrector straightens the sheet curled at the fixing unit so that a straightened sheet is delivered to the reversed-side path.
A sheet that has passed through the fixing unit of a known image forming apparatus is heated to a high temperature. Therefore, even if a curl-corrector is provided to straighten the sheet that has passed through the fixing unit, the sheet may curl again when it passes through the curved guide of the reversing path since the sheet may not be sufficiently cooled while it passes through the curved guide of the reversing path.
To efficiently form images on both sides of a sheet and to increase the productivity of the image forming apparatus, a plurality of sheets of paper should repeatedly be circulated through the delivery path of the image forming apparatus and held still at a predetermined time interval so as to minimize the time interval between the formation of an image on each sheet. To repeatedly circulate and hold still a plurality of sheets of paper for image formation, the sheets of paper must be held still at the curved guide of the reversing path. However, the sheet held still at the curved guide of the reversing path curled even more.
When such a curled sheet is delivered through the image forming apparatus, the sheet cannot smoothly enter the nip between the delivery rollers in the reversed-side path. Therefore, the front edge of the sheet cannot be accurately disposed against the delivery rollers to correct skew. Consequently, the detection accuracy of the horizontal-registration detection unit is reduced and sheets of paper easily become jammed in the delivery path.
To avoid such problems, the curvature of the curved guide of the reversing path may be decreased so that the sheets of paper that pass through the guide path curl less. However, reducing the curvature of the curved guide of the reversing path causes an increase in the size of the reversing path and thus causes an increase in the size of the overall image forming apparatus.
The detection accuracy of the image-detection unit may be increased by providing backup rollers that straighten the sheets of paper by pressing against the sheet. However, to provide such backup rollers, the structure of the image forming apparatus becomes complicated and the production cost increases.